Field of the Invention
The invention concerns an optimization of the noise development of a 3D gradient echo sequence in a magnetic resonance system.
Description of the Prior Art
Magnetic resonance (MR) is a known modality with which images of the inside of an examination subject can be generated. Expressed in a simplified manner, the examination subject is positioned inside a magnetic resonance apparatus in a relatively strong, static, homogeneous basic magnetic field (also called a B0 field) with field strengths from 0.2 Tesla to 7 Tesla or more, such that the nuclear spins of the examination subject orient along the basic magnetic field. To trigger nuclear magnetic resonances, radio-frequency excitation pulses (RF pulses) are radiated into the examination subject, the triggered nuclear magnetic resonance signals are measured (detected) as data known as k-space data, and on the basis of the k-space data MR images are reconstructed or spectroscopy data are determined. For spatial coding of the measurement data, rapidly switched magnetic gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored as complex numerical values in a k-space matrix. An associated MR image can be reconstructed from the k-space matrix populated with values, by a multidimensional Fourier transformation.
A preferred pulse sequence for excitation and acquisition of the magnetic resonance signals is known as a gradient echo sequence, in particular to acquire three-dimensional (3D) data sets. However, such gradient echo-based MR examinations are generally very loud, and therefore uncomfortable for the patient being examined. The main reason for the high noise development is the gradient configurations rapidly changing over time, and the high slew rates (chronological change of the gradient amplitudes dG/dt) associated therewith. Moreover, parameters that require particularly fast switching of the gradients (for example echo times or gradient spoiling) are often required in the protocol of the sequence.
In spite of this, non-selective 3D gradient echo measurements are often used in clinical imaging, but primarily for what are known as preparation measurements. The most important example of such a preparation measurement is a coil sensitivity measurement, which is implemented at least once before beginning the actual patient examination (diagnostic data acquisition) of each patient, if an acquisition coil is used that has a sensitivity profile that is different depending on use. During the coil sensitivity measurement, two 3D measurement data sets are acquired, one measurement data set being acquired with a coil or antenna known as a “body coil”, which is integrated into the magnetic resonance system, and the other measurement data set is acquired with a local coil to be used. The sensitivity profile of the local coil (and therefore the intensity distribution of the local coil image) can be calculated on the basis of these two measurement data sets (which in particular include a division of the two images reconstructed from the respective measurement data sets), and inhomogeneities that arise due to the relative distances of the measurement subject to be examined from the coil element or elements of the local coil can be corrected.
In order to decrease the noise volume of such measurements, the maximum gradient performance provided by the sequence can be decreased until the measurement is markedly quieter. However, the minimum echo time thereby increases, the maximum bandwidth may possibly decrease, and the repetition time TR of the sequence and the measurement time increase, such that the result of the measurement is not optimal under the circumstances.